The present invention relates to coatings, more particularly to methods and apparatuses for measuring passage of chloride ions across a coating such as a marine coating used to prevent or reduce corrosion of a ship hull or other vessel hull.
The present invention was motivated in part by the United States Navy's objective to develop a greater understanding of electrolyte ion kinetics through Navy marine coating systems. The Navy wishes to optimize the thicknesses of marine coating systems for aluminum substrates. Additionally, cost and weight reductions may be achieved by optimizing coatings for aluminum rather than steel substrates. Corrosion represents an enormous burden on the Fleet in terms of life cycle cost, availability, and reliability. Coatings are an integral part of the Navy's strategy to control corrosion.
Chloride ions and dissolved oxygen play distinctive roles in the corrosion of metal substrates. It is of great importance for corrosion mitigation technologies such as coatings to be able to protect a metal substrate from these ions and molecules. Coatings are vital to the Navy's continuing endeavor to prevent or reduce corrosion with respect to a variety of material substrates, coating functions, and service environments. Advancements in coatings may result from improved understanding of coating properties and functions, such as involving adhesion to a substrate, mechanical properties of a membrane, and barrier transport properties of a membrane.
The coating thickness requirements established by the Naval Sea Systems Command (NAVSEA) Standard Item 009-32 was developed for steel alloys. Steel substrates require 16-20 mils of coating for a typical coating system by this standard. However, some ship hulls (e.g., the Littoral Combat Ship (LCS) class) are made of aluminum alloys. Of particular interest to the Navy is the possibility that marine coating system thicknesses for aluminum substrates may be optimized by being decreased (e.g., in terms of mils) in comparison to marine coating system thicknesses for steel substrates.
Aluminum alloys have a passive layer that aids in its protection, where steels do not demonstrate passivity. Therefore, as it currently stands, NAVSEA Standard Item 009-32 may be overly conservative for aluminum substrates, as it has been optimized for steel substrates. Coatings aid in the protection of aluminum because aluminum alloys are subject to localized corrosion wherever the passive film fails. This is especially true in marine environments rich in chloride ions that attack the passive film and initiate pitting corrosion.
As evidenced by a dearth of pertinent information in the literature, coating thicknesses for optimal barrier properties against chloride and other ions have apparently not been thoroughly investigated. Conventional methods for testing marine coatings and their barrier properties are not entirely effective, as these methods involve testing of a coating that is situated on a substrate. Coatings barrier properties have traditionally been examined by electrochemical impedance spectroscopy (EIS). In the early 1990s, scientists such as J N. Murray examined barrier properties of coating systems using EIS.
For instance, Murray et al. mapped disbondment rates of coatings of variable thicknesses on steel to EIS parameters. The coating disbonded area was shown to increase with a decrease in coating thickness. However, no direct correlation of any specific species transport through the coatings was discussed in the literature. Transport rates of water and dissolved oxygen were mentioned as important but difficult to characterize due the interference of the steel substrate.